Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
Because the disk is rotated at a constant angular velocity, the data rate is typically increased toward the outer diameter tracks (where the surface of the disk is spinning faster) in order to achieve a more constant linear bit density across the radius of the disk. To simplify design considerations, the data tracks are typically banded together into a number of physical zones, wherein the data rate is constant across a zone, and increased from the inner diameter zones to the outer diameter zones. This is illustrated in FIG. 1, which shows a prior art disk format 2 comprising a number of data tracks 4, wherein the data tracks are banded together in this example to form three physical zones from the inner diameter of the disk (ZONE 1) to the outer diameter of the disk (ZONE 3). The prior art disk format of FIG. 1 also comprises a number of servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
The servo sectors 60-6N may be written to the disk using any suitable technique, such as an external servo writer which writes the servo sectors on the disk after the disk is inserted into the head disk assembly (HDA), or using a media writer or stamping technique which writes (or stamps) the servo sectors on the disk prior to being inserted into the HDA. Alternatively, the disk drive may self-servo write the servo sectors using circuitry internal to the disk drive.
Regardless of the technique used to write the servo sectors, each track address is typically written over multiple passes. This is illustrated in the example servo pattern shown in FIG. 1B wherein each track address (TKID) is written over two passes (and trimmed during a third pass). For example, when writing the track address TKID 1, the write element is positioned so as to write the upper half of the track address TKID 11 during a first revolution. The write element is then moved one half a track in order to write the lower half of the track address TKID 12 during a second revolution. The write element is then moved another half track in order to trim the lower half the track address TKID 12 while writing the first half of track address TKID 21. As can be seen in FIG. 1B, this servo track writing process results in four seams (e.g., seams X1, X2, X3, and X4) for every two servo tracks written. At each seam an erase band manifests due to the fringe field emanating from the write element which degrades the quality of the written servo data.
There is, therefore, a need to ameliorate the undesirable effect of erase bands in the servo data of a disk drive.